Enable -mpopcnt and -mlzcnt on AVX2
Enable -mpopcnt and -mlzcnt on AVX2

When -mpopcnt or -mlzcnt are explicitly enabled or disabled
(when using -march=native for example), kvazaar builds fail
on older CPUs without support for these flags (see
https://github.com/ultravideo/kvazaar/issues/228 and
https://bugs.gentoo.org/739776 ).

Ensuring these flags are on as done with the rest of AVX2 flags
solves the compilation issues, although it may be a better
approach to provide a configuration option so that only the
specific version matching the build system supported flags is
created in such cases.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar